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I came home one day from work with my somewhat new macbook to discover that Lion was no longer able to connect to the internet. After many hours of troubleshooting I was left with the following...

I can connect to my wireless router okay. My router is connected to the internet just fine (I can ping any domain from my router's console). Lion's browsers and terminal cannot ping or connect to any domain or IP. I run Parallels inside of Lion and my Windows 7 virtual machine can connect to the internet just fine (this is how I'm writing this). When I take my macbook to work the wireless and wired (usb adapter) internet works flawlessly. I've checked to make sure my router hasn't blocked my mac or ip address and reset the router many times. I've confirmed with my internet provider that they do not have any kind(s) of blocks on my account (should be obvious as any other OS on any hardware can connect perfectly fine). I have the most recent versions and updates of all software installed in Lion and Windows.

When I ping an IP I get the following...
request timeout for ICMP_seq(n)
no route to host

When I try to get any information from netstat, it just spins and spins and tries and tries for ages and doesn't give me anything, all while Activity Monitor is showing tons of traffic coming in and going out through the network (from Windows).

It started around the time I installed CentrifyDC, because Lion refused any attempt to bind to my work domain. I use this is my work computer and I need to be able to access network resources. I've talked with my fellow geeks in IT at work about it and they have no idea as they all only know Windows.

So I'm left with this being a routing problem, caused by some file that was created, modified, or deleted during the CentrifyDC installation. (Yes, I uninstalled CentrifyDC).

A further issue is that I have bootcamp installed and therefore Lion will not reinstall because of that. I have a TimeMachine backup, but it is currently on a disk that only my Windows machine can read. Lion refuses to acknowledge the HFS+ partition that Windows can read and write to (I have paragon installed).

I feel like my only real option is to completely format the entire drive, go buy another external hard drive, pray I can get my backups back up, and reinstall everything. I'd really rather not have to go through all that trouble if I could fix this with one line in Terminal...

I haven't felt this helpless in a long time. Can someone please at least point me in the right direction? Please?

You may have already tried it but make sure your network settings are set to automatic.

You can also reset your network settings:
- disconnect from all networks (wifi or cabled)
- in Finder go to /Library/Preferences/SystemConfiguration
- Rename preferences.plist to preferences.plist.old (in case you need to go back to the current configuration)
- restart your Mac and try to connect to your router
- test internet connection etc

Post back with how it goes.

Please use the reputation system if you think you've been helped - bottom left of this post

The solution was, as I expected, that my default gateway was not actually what Lion was reporting and had been changed to my work's default gateway as a static route placed in my routing table by some process that was installed or modified by CentrifyDC.

I went to terminal and did "netstat -rn" and found that indeed my gateway was set to my work's Domain Controller. I did "sudo route flush" to get rid of it, and did "sudo route add default 192.168.1.1" to fix the route. That lets me get to the internet.

But something does keep changing it back, and whenever I want to use the internet I have to do those same commands. So some process keeps changing my default route.

Does anyone know how I can monitor what process is changing my routing table so I can stop it from happening in the future?

I just checked my preferences.plist and discovered the route was in there, so I did as you said and renamed the preferences.plist file, went back into system preferences > Network, setup a new location, and everything was good.

Would it have fixed it if I had just changed my "location" in the preferences earlier?